Field of the Invention
This invention relates to a process for recovering dewaxing aids. More particularly, this invention relates to an improvement in a process for solvent dewaxing a wax-containing hydrocarbon oil containing a dewaxing aid wherein said process produces wax containing said dewaxing aid, the improvement which comprises vacuum distilling said wax to recover a wax bottoms or residue containing said dewaxing aid and recycling said wax bottoms back into the dewaxing process. Still more particularly this invention relates to a process for recovering and recycling dewaxing aids comprising .alpha.-olefin copolymers, chlorinated paraffins/naphthalene condensation products and mixtures thereof by vacuum distilling slack wax containing said dewaxing aids produced via a solvent dewaxing process to about 5-10 wt.% wax bottoms at a temperature no greater than about 320.degree. C. and then recycling the dewaxing aid-containing wax bottoms back into the solvent dewaxing process by adding same to the wax-containing oil feed in an amount ranging from about 0.5-5 wt.%.
Waxes contained in hydrocarbon oils such as lubricating oil fractions are generally removed by solvent dewaxing processes. As the dewaxing solvent, there are generally used low-boiling-point hydrocarbons such as propane, butane, pentane and hexane, and ketone/aromatic hydrocarbon mixtures such as methylethyl ketone/toluene and acetone/benzene. It is known that the treatment capacity of such solvent dewaxing process depends mainly on the filtration rate or efficiency at the filtering step subsequent to the chilling step, and the filtration rate is greatly influenced by the crystal structure of the wax to be separated. In propane dewaxing processes the waxy oil is chilled by the latent heat of evaporation of liquid propane which produces a so-called shock chilling effect causing the formation of fine wax crystals which result in a relatively slow rate of filtration of the dewaxed oil from the wax crystals because of the tendency of the wax crystals to clog the pores of the wax filter cloth in the wax filters and form a dense, relatively impervious mat.
As means for overcoming the foregoing disadvantages, substances capable of changing the size and shape of wax crystals are used so as to facilitate and improve the separation and filtration of waxes at the dewaxing step. These substances are called dewaxing aids and are incorporated together with a solvent into a wax-containing oil. Known dewaxing aids include a chlorinated paraffin/naphthalene condensate sold as a pour point depressant under the tradename of Paraflow, mixtures of an ethylene-vinyl acetate copolymer and a copolymer of unsaturated esters of aliphatic alcohols with acrylic acid or methacrylic acid, .alpha.-olefin copolymers and the like.
The amount of dewaxing aid added to a wax-containing oil is determined after due consideration of such factors as properties (the wax content and viscosity) of the wax-containing oil, the properties of the dewaxing aid, the chilling rate and the filtration rate, but in general, the amount of the dewaxing aid used will range from about 0.01 to 5.0 wt.% of the waxy oil.
However, it has not been clarified whether the dewaxing aid added to the wax-containing oil for attaining the above effect is left in the separated wax component or in the dewaxed oil after the dewaxing treatment. Accordingly, a method for separating the dewaxing aid effectively has not been established, and at the present, the added dewaxing aid is consumed away. Only with respect to the case where an aromatic polymeric substance such as Paraflow is used as the dewaxing aid, there has been proposed a process in which an alcohol is incorporated in slack waxes to extract and to precipitate the polymeric substance contained therein and a process in which slack waxes are passed through an adsorbing bed to separate the polymeric substance by adsorption. These processes, however, are defective in that the extracting solvent or adsorbent must be generated, the process steps are complicated and there is a risk of loss of the dewaxing aid.
As a result of various investigations, we found that the dewaxing aid added cocrystallizes with the wax when the wax is precipitated from a wax-containing oil at the chilling step to thereby attain the dewaxing performance, and the effective ingredient of the dewaxing aid is completely left in the separated slack waxes.
Based on this finding, we furthered our researches with a view to developing a process in which the effective ingredient of the dewaxing aid can be recovered substantially completely from separated slack waxes and the recovered effective ingredient is added to the wax-containing oil feed again to conduct the dewaxing step repeatedly.